Resin is a substance with large chain-like molecules formed by connecting together small molecules (called monomers) in a reaction of polymerization. Reaction of polymerization may be initiated by mixing two parts of resin together (for two-part epoxy or polyurethane resins, for example) or by exposing the resin to UV-light (UV resins).
Many types of resins are used in crafts. Clear transparent resins work well with polymer clay to create an extra-durable glass-like surface coating.
Here is an example of a pendant created using polymer clay and resin in my Faux Cloisonne technique.
See also: EnviroTex Lite and ICE Resin.